Conventionally, a magnetic tape has been mainly used as a recording medium on which a coded information signal such as image and audio is recorded. In recent years, however, recording media of different types, such as an optical disk, a hard disk and a semiconductor memory, are increasingly often used as the recording medium. In the magnetic tape, a recordable data volume per unit time is fixed due to its structural restrictions, in response to which the image and audio are also to be coded based on a fixed rate. In contrast, in any of the optical disk, hard disk and semiconductor memory, a recordable data volume per unit time is optionally determined as far as the recording is carried out to any of the media at a maximum recording speed thereof or below. Therefore, when the image and audio to be recorded on any of the media are coded, a relevant coding amount can be changed depending on the complexity of the image and audio. In other words, a generally called variable rate is available in such a medium.
When the variable rate is employed as the coding amount in the coding process, the coding amount to be allocated to the information signal of a high complexity can be increased, while the coding amount to be allocated to the information signal of a less complexity can be reduced. In the foregoing manner, when the variable rate is compared to the fixed rate based on the same total coding amount, an overall quality in using the variable rate can be improved.
As another trend, an increasing number of information transmitting devices for coding the information signal such as image and data and transmitting the information signal to a different device via a transmission line such as LAN are available in recent years. When a recording medium having a large capacity is used as a memory for temporarily memorizing transmission data, the transmission can be carried out based on the variable rate to a certain extent though the transmission line has a limited number of transmission bands.
As an example of a variable rate control implemented in the information recording device for coding and recording the information signal such as image and audio, a methodis disclosed in No. 3375851 of the Japanese Patent Applications. In the method recited in the document, recording data is temporarily recorded in a temporary memory device and thereafter actually recorded. More specifically, the data stored in the temporary memory device is immediately written in the recording medium during the time when the coding rate continuously stays below a maximum value of a rate at which the data of the memory device can be recorded on the recording medium (hereinafter, referred to a maximum recoding rate) by a recorder. On the contrary, as soon as the coding rate starts to exceed the maximum recording rate, the data which has not been recorded yet is stored in the temporary memory device as recording-standby data, and then written in the recording medium. In the process, an available capacity of the temporary memory device is monitored so that the coding amount is controlled in such manner that the coding amount exceeding the available capacity of the temporary memory device is not allocated.
However, when the available capacity of the temporary memory device continuously remains small, the coding amount to be allocated to the information signal of a higher complexity cannot be increased. As a result, the coding amount flexibly corresponding to the complexity of the information signal cannot be allocated, which remarkably deteriorates a quality of the coded information signal, in particular, the quality of a high-complexity part.